Hydrothermal Synthesis and Characterization of Litchi

Journal of The Electrochemical Society, 165 (9) E303-E310 (2018)

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Hydrothermal Synthesis and Characterization of Litchi-Like NiCo2 Se4 @carbon Microspheres for Asymmetric Supercapacitors with High Energy Density Yue Li, Lanshu Xu, Mengying Jia, LinLin Cui, Jianmin Gao, and Xiao-Juan Jin

z

MOE Key Laboratory of Wooden Material Science and Application, Beijing Key Laboratory of Lignocellulosic Chemistry, MOE Engineering Research Center of Forestry Biomass Materials and Bioenergy, Beijing Forestry University, Haidian, 100083 Beijing, People’s Republic of China In this paper, litchi-like microspheres structures of NiCo2 Se4 @carbon microspheres (NCSC) hybrids were successfully synthesized via a simple two-step hydrothermal method. The effects of the content of NiCo2 Se4 on the structure of NCSC composites are investigated. The morphology and nanostructure of the as-obtained NCSC hybrids were examined by XRD, XPS, FESEM and TEM. The results indicate that the carbon microspheres (CM) were uniformly covered with NiCo2 Se4 and with the increase of NiCo2 Se4 content, the superficies of NCSC also exhibits homogeneous rough nanoparticles and regular spheres connect with each other. Meanwhile, the electrochemical tests indicate a suitable mass ratio of NiCo2 Se4 and carbon microspheres (2:3) showed the highest specific capacitance value (1394 F g−1 at the current density of 0.5 A g−1 , 766 F g−1 even at the current density of 50 A g−1 ), excellent rate property and outstanding cycling stability (80.093% of the initial value after 10000 cycles at the current density of 5 A g−1 ). Benefiting from the above integrated advantages, asymmetric supercapacitors (ASC) fabricated with NCSC composites as the positive electrode and porous activated carbon as negative electrode operating at the voltage window of 0–2 V deliver a high energy density of 101 Wh kg−1 at a power density of 0.749 kW kg−1 , with the specific capacitance retention of 81.095% after 10000 cycles at the current density of 10 A g−1 , superior to most of reported supercapacitors. © The Author(s) 2018. Published by ECS. This is an open access article distributed under the terms of the Creative Commons Attribution Non-Commercial No Derivatives 4.0 License (CC BY-NC-ND, http://creativecommons.org/licenses/by-nc-nd/4.0/), which permits non-commercial reuse, distribution, and reproduction in any medium, provided the original work is not changed in any way and is properly cited. For permission for commercial reuse, please email: [email protected]. [DOI: 10.1149/2.0991807jes]

Manuscript submitted March 21, 2018; revised manuscript received May 3, 2018. Published May 24, 2018.

While batteries keep our light devices operating throughout the day, they take hours to recharge. For rapid power delivery and recharging, electrochemical capacitors, also called supercapacitances, hold great promise.1 Besides the fast rate of charge and discharge, supercapacitances also exhibit admirable reliability, long cycling lifespan, excellent power density and the ability to bridge the energy-power gap between conventional Li-ion batteries and capacitance.2,3 Based on the use of electrode materials and charge storage mechanisms, supercapacitances can be divided into two categories, electrochemical double-layer capacitances (EDLCs) and redox pseudocapacitors.4,5 EDLCs store energy via formation of the electrical double layer at the interfaces between the electrodes and electrolytes. Carbon materials are usually used as the electrode materials of EDLC. Unfortunately, EDLCs could not meet the ever-growing need for peak-power assistance in electric vehicles due to the low specific capacitance and energy density.6,7 By contrast, the faradaic pseudocapacitors could significantly increase the specific capacitor and energy density by using the fast and reversible surface and near-surface redox reactions within the active materials.8–11 Within the multifarious pseudocapacitive electrode materials, RuO2 , presenting the noticeably high specific capacitance (as high as 760 F/g) and high energy density, has been considered as the best electrode materials for pseudocapacitors. However, the high cost of this precious metallic oxide hinders its practical application.12–14 So far, various sorts of electrode materials have been investigated for pseudocapacitances, including conducting polymer,15 transition metal oxide,16 hydroxides and sulfides and carbon materials.17–19 have been investigated systematically as electrode materials for supercapacitances. However, most of pseudocapacitive electrodes suffer from poor electronic conductivity, low cycling stability and low high-rate specific capacitances12,20 which seriously hinder the rate capability of pseudocapacitive materials and their practical applications. Therefore, it is imperative to search for a promising pseudocapacitive electrodes materials with prominent properties. The properties of supercapacitances rely chiefly on the performances of the active electrode materials that can be divided into three categories, consisting of transition metal oxides, conductive polymer

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E-mail: [email protected]

and carbon materials.21,22 Among them, transition metal based chemical compound, containing hydroxides, oxides, sulfide and nitride. Very recently, ternary nickel cobalt oxide (NiCo2 O4 ) and ternary nickel cobalt sulfide (NiCo2 S4 ) have drawn tremendous attention due to its superior electrochemical activity, richer electrochemical redox reactions compared with binary nickel oxide (NiO), cobalt oxide (Co3 O4 ), nickel chalcogenide (NiS) and cobalt chalcogenide (CoS).23–32 Very recently, Haichao Chen and his group have demonstrated that the NiCo2 S4 , used as pseudocapacitive materials, has exhibited much better conductivity and rather higher electrochemical properties than NiCo2 O4 .33 Consequently, ternary Ni-Co chemical compounds exhibit eminent latent used as pseudocapacitive materials for excellent properties energy storage devices. This is mainly attributed to the fact that NiCo2 O4 (NiCo2 S4 ) possess multiple oxide states that enable rich redox reactions originating from both nickel and cobalt ions. Besides, NiCo2 O4 (NiCo2 S4 ) own many inherent merits, for instance, abundant resources, low cost and environmental friendliness. All these outstanding properties enable NiCo2 O4 (NiCo2 S4 ) become a potential candidate for electrode materials. Besides NiCo2 O4 , NiCo2 S4 ,34–36 another type of ternary Ni-Co compounds, NiCo2 Se4 , has shown much higher conductivity and lower optical bandgap compared with NiCo2 O4 (NiCo2 S4 ) Owing to the electronegativity of the selenium is lower than that of sulfur and oxygen, which could avoid the disintegration of the structure by the elongation between layers and enable it convenient for electrons to transport in the structure, the replacement of sulfur and oxygen with selenium could be an advisable choice. Accordingly, as a fancy type of materials, NiCo2 Se4 is of vital research value. For supercapacitance, a few single metal selenide-based electrodes show great promising for energy storge. For instance, Hui Peng and his group have prepared a novel asymmetric supercapacitor using the petallike cobalt selenide (Co0.85 Se) nanosheets as positive electrode in a 2 M KOH aqueous electrolyte. The assembled asymmetric supercapacitor device possesses an extended operating voltage window of 1.6 V, high power density of 400 W kg−1 at a energy density of 21.1 Wh kg−1 .18 Haichao Chen et al. have synthesized the bimetallic Ni0.67 Co0.33 Se with the specific capacity values of 535 C g−1 at current density of 1 A g−1 and the capacity retention values are 62% after 50 times increasing in current density.33 Therefore, using NiCo2 Se4 as

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Figure 1. Schematic illustration of the fabrication process of NCSC.

electroactive materials for supercapacitance seems impressive. However, the entire properties of the assembled supercapacitance could be much moderate owing to the low content of active materials, including the weight of the current collector especially. Rationally devising the electrode materials, including the construction of active materials, is extremely essential to achieve high energy density and power density for supercapacitance.37,38 Carbon-based materials, especially porous CM is one of the best choices for construction of excellent- properties electrode materials as supporters owing to excellent conductivity, easy processability, inert to corrosion, higher abundance, lower cost and great thermal and mechanical performances.39 Furthermore, CM has enormous specific surface area and pore texture, which provides it as striking substrate materials when synthetized with transition metal compounds and conductive polymers for supercapacitance, lithium ion cells and others. CM could reduce the resistance of electrolyte diffusion and increase the capacity of the electrode owing to the regular globular. Furthermore, the interspace among the CM could enable easy for electrolyte access to electrodes, which is favorable to the formation of EDLC between the carbon and electrolyte interface.40 Successful assembled with Co3 O4 , MnO2 , NiO, polypyrrole and polyaniline has been investigated.29,31,41 In the present work, we have synthesized the NCSC hybrid using a simple hydrothermal process as a novel pseudocapacitive active material for supercapacitance. The nanostructured ternary nickel cobalt selenides were firsthand anchored on CM avoiding the addition of polymer binders and conductive agents that are adverse to excellentproperty supercapacitors. Beyond that, directly anchored on carbon microspheres could reduce the contact resistance between the carbon microspheres collector and electroactive materials which could promote the electron transfer.42,43 The ultrathin fluffy slices of NiCo2 Se4 uniformly decorated the CM which could significantly enhance the specific capacitance. CM as the current collectors and supporters could increase the conductivity and stability. By taking advantage of the merits of both, the NCSC exhibited outstanding properties when used as the supercaoacitor electrode material.

Experimental Material preparation.—Fabrication of NiCo2 Se4 .—All of the chemicals were of analytical grade and used directly without further purification. The formation process of sample is depicted in Fig. 1. For

a typical run, firstly, NiCl2 •6H2 O (0.476 g), CoCl2 •6H2 O (0.952 g) and ethylenediamine (5.2 ml) were dissolved in mixed solution of ethanol (60 ml) and water (20 ml) and vigorous stirred until to form homogeneous clear pink solution. Afterward, Na2 SeO3 (0.4 g) and hydrazine hydrate (15 ml) were added to the homogeneous solution, stirred for about 1.5 h, transferred into a Teflon-lined stainless steel autoclave (100 ml) and heated in an oven at 180◦ C for 12 h. After the autoclave was cooled down to the room temperature, the precursor was obtained by filtration and rinsed several times with ethanol and deionized water, respectively, and then dried at 60◦ C for about 12 h. Fabrication of NCSC.—For a typical run, carbon microspheres was treated by mixed acid to purify and make it hydrophilic first CM (0.15 g) and NiCo2 Se4 (0.1 g) was added in mixed solution of ethanol (60 ml) and water (30 ml) and vigorous stirred for about 10 h to obtain the homogeneous solution in sealed condition. Afterward, the mixture was transferred into a Teflon-lined stainless steel autoclave (100 ml) and heated in an oven at 180◦ C for 24 h. After the autoclave was cooled down to the room temperature in the air. the precursor was obtained by centrifugation and rinsed several times with ethanol and deionized water, respectively, and then dried at 60◦ C for about 12 h. During the procedure, the mass ratio of NiCo2 Se4 and CM were 1:2, 5:9, 2:3, 5:6 and 1:1, respectively. And the synthesized hybrids were mark NCSC 1:2, NCSC 5:9, NCSC 2:3, NCSC 5:6, NCSC 1:1. Material characterization.—The JSM-7001F field emission scanning electron microscope (FESEM, Japan) and the TecnaiTF20 Transmission electron miscroscopy (TEM, Netherlands) was used to investigate the morphologies and microstructure. Energy-dispersive spectroscopy (EDS) element analysis were also performed on the same instrument in TEM mode. X-ray diffraction (XRD) spectra were collected on D/max-2550 diffractometer with Cu kα- 1radiation (λ = 0.15406 nm). The data of X-ray photoelectron spectroscopy (XPS) were recorded on Kratos Analytical Ltd by using Al Kα, hv = 1486.7 eV. Electrochemical measurements.—Electrodes used for fabrication of symmetry supercapacitors was prepared by mixing the NCSC, acetylene black and polytetrafluoroethylene (PTFE) with ethanol in a mass ratio of 87:10:3. Next, the mixture was pressed onto a nickel foam substrate and then dried in an oven at 105◦ C for over 4 h. The electrochemical performance of the NCSC samples were evaluated



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Figure 2. Morphology and structure of the samples: typical overview FESEM and TEM images of (a) NCSC 2:3 and (d) NCSC 1:1, and magnified FESEM images of (b-c) NCSC 2:3 and (e-f) NCSC 1:1. TEM images of (g-h) NCSC 2:3 and (i) NCSC 1:1. (j) FESEM image of the NCSC 2:3 electrode and the corresponding EDS mapping (area distribution of C, Ni, Co, Se elements) of the NCSC 2:3.

using the CHI 660D electrochemical workstation (Solartron Metrology, UK) and BT2000 battery testing system (Arbin Instruments, USA). The capacitive properties of all NCSC samples were evaluated in 7 M KOH aqueous electrolyte using two-electrode cells. The specific capacitance of single electrode was calculated from galvanostatic charge-discharge curves according to the following equations: C = (I t)/mV

[1] −1

Where C is the specific capacitance of single electrode (F g ), I is the discharge current (A), t the discharge time (s), m is the mass of the active material (g) and V is the potential window during the discharge process except the ohmic drop (V). The asymmetric supercapacitors were assembled with the NCSC hybrid as positive electrode, activated carbon as the negative electrode and polypropylene diaphragm paper as the separator. On the negative side, we used the mixture prepared by mixing the porous activated carbon, acetylene black and polytetrafluoroethylene (PTFE) with ethanol in a mass ratio of 87:10:3. The mixture was treated at 105◦ C for over 4 h and then was pressed onto nickel foam. The electrochemical measurements of the fabricated NCSC//activated carbon asymmetric supercapacitor were implemented in 7 M KOH aqueous electrolyte in a two-electrode cell at room temperature. The power density (P) and energy density (E) of the asymmetric supercapacitors

were calculated according to the following equation: Cm = (I t)/(MV )

[2]

E = (Cm V 2 )/2

[3]

P = E/t

[4]

Where C is the specific capacitance of NCSC//activated carbon asymmetric supercapacitor (F g−1 ) and M is the total mass of active materials on both electrodes, V is the voltage range after ohmic drop (V). Results and Discussion The field emission scanning electron microscope (FESEM) and transmission electron microscopy (TEM) were used to examine the morphologies and microstructure of the obtained samples. The panoramic and close-up images were represented in Figs. 2a–2f and Figs. 2g–2i. The mixed-acid treated carbon microspheres used in this paper have relatively smooth surface with a diameter of about 5–100 nm. The carbon microspheres were well coated by NiCo2 Se4 after treating the NiCo2 Se4 composite solution. As represented in


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Table I. The porosity parameters of CM and NCSC. Sample

SBET (m2 /g)

Smi (m2 /g)

Sdft (m2 /g)

Vtot (cm3 /g)

Vmi (cm3 /g)

Vme (cm3 /g)

Wp (nm)

CM NCSC

1586 1434

1499 1326

1692 1588

0.82 0.76

0.66 0.61

0.46 0.42

2.22 2.20

Figs. 2a–2c, from the low-magnification FESEM image (Fig. 2a), it is clearly observed that there is many globular-like NiCo2 Se4 closely packing on carbon microspheres. As shown in Table I, the specific surface area of the CM hybrids (1586 m2 /g) is more than NCSC hybrids (1434 m2 /g). Higher magnification images (Figs. 2b, 2c) exhibit that the carbon microspheres are uniformly decorated with ultrathin fluffy slices, where the mass ratio of NiCo2 Se4 and carbon microspheres is 2:3. However, the NiCo2 Se4 composite are particularly intensive and irregularity, attributing to the excessive of NiCo2 Se4 , Figs. 2d–2f presents the FESEM images of NiCo2 Se4 with carbon microspheres mass ratio of 5:9. With the increase of NiCo2 Se4 content, the superficies of NCSC exhibits much fatter and deliver together, which is disadvantage of electrolyte ion trapping and access to electrochemical active sites. TEM (Figs. 2g–2h) images describe that the NCSC is created by substantial amount of NiCo2 Se4 nanoparticles loosely stacked ultrathin-layer NiCo2 Se4 nanosheets on the appearance of carbon microspheres, where the mass ratio of NiCo2 Se4 and CM is 2:3. Fig. 2i shows that the surface of NCSC is composed of fairly rough which is in favorable agreement with the results of FESEM. The elemental distribution of NCSC composites is examined by energy-dispersive X-ray spectroscopy (EDS) mapping measurement and the result described in Fig. 2j. Evidently, the C, Ni, Co, Se elements are uniformly distributed throughout the observed area, illustrating the successful and homogeneous distribution of the target material. The crystal structure and phase purity of the NCSC composites are further characterized by X-ray diffractometer (XRD), of which the results are described in Fig. 3a. All the diffraction peaks of XRD patterns are excellently indexed to (100), (101), (102), (110), (103) and (112) plane reflections of NiSe (JCPDS no. 75–0610), CoSe (JCPDS no. 70–2870) and graphitic carbon microspheres (JCPDS no. 26–1076) respectively, indicating that the pure phase of the as-obtained samples. The diffraction peak centered at 28.5◦ , which is belonging to carbon microspheres, declares the composite containing the carbon microspheres. With the increase of carbon microspheres content, the diffraction peaks become broad, which is well consistent with the FESEM and TEM in Figs. 2a–2i. To further determine the chemical states, valence and bonding situation in NCSC, the NCSC 2:3 is analyzed by X-ray photoelectron spectra (XPS) as illustrated in Figs. 3b–3f. Fig. 3b shows the survey spectrum of NCSC 2:3 sample. The peaks have been marked, which could disclose the existence of C, Ni, Co, Se and O. The O element comes from the ineluctable the surfaces adsorption of sample due to the exposure of air. Therefore, the composites consists of C, Ni, Co, Se and O, which is in line with our experiment. Figs. 3c–3f represent the C 1s, Ni 2p, Co 2p and Se 3d spectra for NCSC, respectively. As described in Fig. 3c, the main peak centered at 284.3 eV of C 1s spectra which attributed to sp2 hybridized C atoms. For the Ni 2p spectra, as shown in Fig. 3d, the Ni 2p spectra exhibits two major peaks with corresponding satellites. The Ni 2p1/2 and Ni 2p3/2 is situated in 873.45 eV and 855.65 eV and the corresponding satellite peaks of 879.55 eV and 860.749 eV respectively, demonstrating the existence of Ni2+ . The two characteristic peaks of Co 2p1/2 and Co 2p3/2 are centered at 793.35 eV and 778.95 eV with the corresponding satellite peaks of 796.95 eV and 781.3 eV respectively in Fig. 3e, which demonstrating the existence of Co3+ . As described in Fig. 3f, the peaks centered at 58.85 eV and 54.75 eV could be attributed to the Se 3d 3/2 and 3d 5/2 ,44–46 which is representative of metal-selenium bonds. On the basis of the analysis of XPS, the hybrid is composed of Ni2+ , Co3+ , Se2− and C atom, which is consistence with the formula of NCSC. Due to the unique structure with the litchi-lke structure, the NCSC is expected to be an electrode materials of great promise for superca-

pacitor applications. The electrochemical properties of the NCSC with different mass ratios are carried out by galvanostatic charge discharge (GCD), cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The electrochemical performances of NCSC electrodes are researched in 7 M KOH electrolyte and the results exhibited in Fig. 4. Fig. 4a describes the GCD curves for all the electrodes with different mass ratios considered from 0 V to 1 V at the current density of 0.5 A g−1 . The charge curves show asymmetrical with discharge curves and exhibit irregular triangle shape owing to the representative pseudocapacitive faradaic. And the NCSC 2:3, the charge and discharge time has achieved longest, revealing the highest specific capacitance value (1394 F g−1 ) comparing with others kinds of the electrode materials. However, with the increase of ternary nickel cobalt selenides, ternary nickel cobalt selenides are particularly intensive and irregularity that definitely reduce the interspace among the NCSC. Besides, the CM was tightly enclosed that it is hard for CM access to electrodes and cannot exert its excellent conductivity and stability which could reduce the electronic conductivity and seriously impede the specific capacitances. As a typical example, the NCSC 2:3 sample is selected to investigate the electrochemical performances due to the superior specific capacity. Fig. 4b describe the GCD curves of NCSC 2:3 electrode at various current densities. The NCSC 2:3 electrode has achieved the eminent specific capacitances of 1394, 1141, 1074, 977, 862, 781 and 766 F g−1 at the current densities of 0.5, 5, 10, 20, 30, 40 and 50 A g−1 . With the increase of current density, the specific capacitance decreases slowly, demonstrating the electrode allows the swiftly ion diffusion. Fig. 4c depicts the CV curves of NCSC with different mass ratios at a scan rate of 50 mV s−1 in the potential windows of 0–1 V. It is note that all the CV curves contains a palpable broad redox peaks attributed to the faradaic redox reactions, demonstrating the pseudocapacitive characteristic. Furthermore, the NCSC 2:3 exhibits the largest CV area related to the litchi-like structure arising from the excellent constructing of NiCo2 Se4 and carbon microspheres. The result of CV is in agreement with the result of GCD. Fig. 4d exhibits the homologous CV curves of NCSC 2:3 at various scan rates. Evidently, with the increase of scan rate, the CV area become larger, the spread-dominated devotion becomes weaker, leading to the reduction of specific capacitance at high scan rates. Furthermore, the peaks transfer slightly along with the scan rate, attributing to the reaction capability and ion/electron transfer at the interface of electrode and electrolyte. The specific capacitance of NCSC electrodes with various mass ratios counted according to the GCD curves and summarized in Fig. 4e, notably demonstrating that NCSC 2:3 show an outstanding specific capacitance of 1394 F g−1 at the current density of 0.5 A g−1 , which is higher than 993 F g−1 of NCSC 5:9 and 1388 F g−1 of NCSC 5:6, both at 0.5 A g−1 . What’s more, it is significant that the specific capacitance of the NCSC 2:3 electrode could be achieved 766 F g−1 even at the current density of 50 A g−1 , which much higher than 566 F g−1 of NCSC 5:6 and 712 F g−1 of NCSC 5:9, both at he current density of 50 A g−1 . However, with the increase of mass ratio of ternary nickel cobalt selenides, the surface of NCSC is particularly intensive and irregularity, which could reduce the electronic conductivity and seriously impede the high-rate specific capacitances that is in agreement with the GCD. The cycling stability of the NCSC 2:3 electrode was further investigated by the specific capacitance retention at the current density of 5 A g−1 and the result summarized in Fig. 4f. Noticeably, the specific capacitance of NCSC 2:3 composite electrode could maintain 80.093% of the initial value after 10000 cycles, which is much better than the materials reported previous, demonstrating that the NCSC 2:3 is remarkable for long time electrochemical stability. The striking



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Figure 3. Structure characterization of the samples: (a) typical XRD patterns of the as-obtained NCSC 5:6, NCSC 2:3 and NCSC 1:1. (b) XPS full spectrum; (c) C 1s, (d) Ni 2p, (e) Co 2p, (f) Se 3d for NCSC 2:3.

electrochemical property of NCSC 2:3 electrodes could be attributed to the unique stabilized structure that the globular-like ternary nickel cobalt selenides compound enclosed carbon microspheres furnishes spaces for easily accessible to electrolyte ions, resulting in swift mass transport and the boosted redox reaction kinetics and enhanced the total specific capacitance. In addition, the charge transfer rate and the interface ion absorption-desorption rate were examined by electrochemical impedance spectroscopy (EIS) measurements in the 7.0 M KOH in the frequency range from 0.01 Hz–10 KHz and summarized in Fig. 4g, which exhibits the Nyquist plots of NCSC with various mass ratio. Obviously, all the nyquist plots include an observed straight line in low frequency region which could respond to the diffusion-controlled Warburg impedance, and a similar semicircle in high-frequency region which ascribes to the charge-transfer resistance of the electrodes and electrolyte interface (Rct ). The expanded view of the EIS spectrum is exhibited in the inset of Fig. 4g. Obviously, the NCSC 2:3

exhibits the smallest semicircle among all the nyquist plots of all the electrodes which represents the NCSC 2:3 has the smallest resistance that the characteristic of the fast and easy transfer of electrons and ions into the inlayer of the material. The result demonstrated that the unique nanostructure with ternary nickel cobalt selenides wrapped up the carbon microspheres could reduce the charge transfer resistance owing to the short electron and charge transport pathways and the high surface area. However, as the increase of NiCo2 Se4 content, the semicircle in the nyquist plots have increased, implied that the charge transfer resistance has increased, attributing to the serious accumulation of ternary nickel cobalt selenides which seriously impede the electrochemical conductivity. What’s more, all Nyquist plots of all NCSC show almost vertical line at low frequency demonstrating the ideal capacitive performance, especially for NCSC 2:3. In order to gain a deep insight into the potential for practical application of as-obtained NCSC electrode, a variety of electrochemical measurements of ASC were demonstrated with the NCSC 2:3 as


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Figure 4. Electrochemical property of the NCSC samples with various mass ratios: (a) The GCD curves of NCSC hybrids electrodes with various mass ratios at the current density of 0.5 A g−1 (b) the GCD curves of NCSC 2:3 at various current densities. (c) CV curves of NCSC hybrids electrodes with various mass ratios at the scan rate of 0.1 V s−1 (d) CV curves of the NCSC 2:3 electrode at various scan rates (e) specific capacitance of NCSC hybrid electrodes at different current densities (f) cycling property of NCSC 2:3 at the current density of 5 A g−1 . Inset: The corresponding GCD curves of the first and the last 9 cycles of the hybrid electrode during 10000 GCD cycles. (g) Nyquist plots of NCSC hybrids electrodes in 7 M KOH in the frequency range from 10−2 to 104 Hz. Inset exhibit the close-up view on the high-frequency of various NCSC hybrids electrodes. Downloaded on 2018-05-24 to IP 181.214.30.255 address. Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use) unless CC License in place (see abstract).


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Figure 5. Electrochemical property of the NCSC 2:3//activated carbon ASC: (a) GCD curves at various current densities. (b) CV curves at various scan rates (c) specific capacitance at different current densities (d) Ragone plot for energy density and power density (e)cycling property at the current density of 10 A g−1 . Inset: The corresponding GCD curves of the first and the last 6 cycles of the hybrid electrode during 10000 GCD cycles.


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positive electrode, porous activated carbon as negative electrode (NCSC 2:3//AC) using a two-electrode configuration in 7 M KOH electrolyte. Fig. 5a shows the GCD curves of ASC at the current densities range from 0.5 A g−1 to 50 A g−1 . The GCD curves have no overcharge when the voltage was increased to 2 V, demonstrating excellent electrochemical property of ASC up to 2 V. Furthermore, the curves show nonlinear profile, suggesting that the specific capacitance of ASC storage is mainly attributed to the pseudocapacitance. Fig. 5b exhibits the CV curves of NCSC 2:3//AC at scan rates range from 0.5 V s−1 to 20 V s−1 in the voltage window of 0–2 V. Notably, the redox reaction peaks under various scan rates can observe, the redox peaks could be obvious even at the high scan rate of 2 V s−1 , demonstrating the excellent rate property which is consistent with the GCD curves. According to the GCD curves at various current densities, the corresponding specific capacitance were calculated in view of the entire mass of the active materials of the two electrodes and summarized in Fig. 5c. The NCSC 2:3//AC show a high specific capacitance of 721.5 F g−1 at 0.5 A g−1 , and remains 403.5 F g−1 even at 50 A g−1 . The result is comparable to the date reported previous. In addition, The cycling stability of the NCSC 2:3//AC was further investigated by repeated charge and discharge test at the current density of 10 A g−1 for 10000 cycles, and the result summarized in Fig. 5e. Noticeably, the specific capacitance of NCSC 2:3//AC hybrid supercapacitors could maintain 81.095% of the initial value after 10000 cycles, which is much better than the ASC reported previous, demonstrating that the NCSC 2:3//AC is remarkable for long time electrochemical stability. Furthermore, high specific capacitance and broad voltage window are critical to achieve an efficient supercapacitor with high power density and high energy density based on the Equation. The Ragone curve of NCSC 2:3 for energy density (E, Wh kg−1 ) and power density (P, kW kg−1 ) of NCSC 2:3//AC hybrid supercapacitors in a voltage window of 0–2 V at different current densities is illustrated in Fig. 5. The specific energy density decreased from 101 Wh kg−1 to 70.7 Wh kg−1 on the basis of the ttal mass of active material, nevertheless, the power density has rised from 0.749 kW kg−1 to 38.03 kW kg−1 at the current density ranging from 1 to 50 A g−1 . The NCSC 2:3//AC could achieve a high energy density of 70.7 Wh kg−1 at a power density of 38.03 kW kg−1 at the current density of 50 A g−1 which was much more than those of most reported such as NiCo2 S4 //RGO,47 NiCo2 O4 @CNT//CNT,12 and Ni-Co LDH//RGO,21 Therefore, the NCSC 2:3//AC is capable for high power and energy device. Notably, all the measurements demonstrate that the NCSC as a positive electrode for advanced ASC. Conclusions Litchi-like NCSC hybrids with various NiCo2 Se4 /carbon microspheres mass ratios were successfully synthetized via a facile two-step hydrothermal method. Among all the as-obtained samples, the NCSC 2:3 hybrids delivers the best electrochemical property. The NCSC 2:3 electrode showed the highest specific capacitance value (1394 F g−1 at the current density of 0.5 A g−1 , 766 F g−1 even at the current density of 50 A g−1 ), excellent rate property and outstanding cycling stability (80.093% of the initial value after 10000 cycles at the current density of 5 A g−1 ). Meanwhile, the as-obtained NCSC 2:3//AC could achieve a high energy density of 70.7 Wh kg−1 at a power density of 38.03 kW kg−1 at the current density of 50 A g−1 , as well as the outstanding cycling stability with 81.095% of the initial value after 10000 cycles. The work exhibited that the NCSC hybrids is a promising electrode material for the energy storage. Acknowledgments The authors gratefully acknowledge the financial support of National Natural Science Foundation, project (51572028). ORCID Xiao-Juan Jin

https://orcid.org/0000-0001-7085-2514

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